We derive an expression relating the transport parameter $\widehat{q}$ and the shear viscosity $\eta$ of a weakly coupled quark-gluon plasma. A deviation from this relation can be regarded as a quantitative measure of “strong coupling” of the medium. The ratio $T^3/\widehat{q}$, where $T$ is the temperature, is a more broadly valid measure of the coupling strength of the medium than $\eta /s$, where $s$ denotes the entropy density. Different estimates of $\widehat{q}$ derived from existing Relativistic Heavy Ion Collider data are shown to imply radically different structures of the produced matter.

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